Conventionally, there has been proposed a technique of forming a projecting portion on a ground electrode of a spark plug through press working, in order to improve ignition performance of the spark without use of a noble metal chip on the ground electrode. Japanese Patent Application Laid-Open (kokai) No. 2006-286469 (Patent Document 1) discloses a technique of forming a projecting portion of a ground electrode by means of “press forging,” which is one type of press working. Shin Nishioka et al., “Super Ignition Spark Plug with Wear Resistive Electrode”, SAE TECHNICAL PAPER SERIES 2008-01-0092, published in April 2008 (Non-patent Document 1) discloses a technique of forming a projecting portion on a ground electrode by means of “half punching,” which is another type of press working. Specifically, a ground electrode is pressed from the upper side thereof with a working pin so as to form a recess portion, to thereby form a projecting portion on the side opposite the recess portion.